Ralf Buchner

Pollen‐connecting threads in Gymnocalycium (Cactaceae): Their origin, function, and systematic relevance

In dehisced anthers of all Gymnocalycium species (Cactaceae, subfam. Cactoideae, tribe Trichocereae) investigated so far, a small percentage of pollen grains are aggregated by only a few threads. These threads are located at the margin of the dehiscing zone of stomium and septum. They are not part of the pollen grain wall, and they are not resistant to acetolysis, thus indicating the absence of sporopollenin. The threads originate in part from modified stomium/septum cells (split cell walls, cytoplasmic remnants and lipid droplets) and in part from pollenkitt produced by the—now desintegrated—tapetal cells. This highly viscous, gum‐like substance along the modified stomium and septum cell walls may assume an irregular thread‐like habit before or during dehiscence. Pollen aggregates may be formed frequently by different sticky or non‐sticky agents. A detailed conspectus on origin, nature, significance and function of various types of pollen‐connecting agents forming threads is presented. Pollen‐connecting ...

Pollen of limnanthes douglasii a reinvestigation

Abstract The pollen of Limnanthes douglasii was reinvestigated in order to clarify certain disagreements. Contrary to former authors the (single) tetrahedral/decussate tetrad and the microspore orientation is interpreted as follows: the ring-like aperture region of the spherical monads/young microspores runs parallel to the equator, but is slightly shifted towards the distal face. A smaller polar area is found distally, while a larger one is located proximally. As a result of our interpretation of the pollen orientation, the pollen grains must be described as zonosulcate. The unusual arrangement of relatively thick and thin intine regions gives rise to a quite uncommon harmomegathic effect. Dry pollen is hooked, while expanded pollen is irregularly ellipsoidic or kidney-shaped. The unique pollen characters suggest an isolated taxonomic position of this family.

Development and structure of the comose seeds ofHillia (Rubiaceae)

The hairs at the apical end of the seeds ofHillia are pluriseriate, multicellular structures. The cells making up a hair are elongated exotesta cells and, consequently, also have secondary thickenings identical (H. parasitica) or similar (H. costanensis) to those found on the exotesta cells on the main body of the seeds. Hair formation already starts in bud stage: at and around the chalazal region of an ovule, integument epidermis cells are grouped together to form ± elongated packets of 4–7 cells. The cells of each packet undergo further elongation and anticlinal division so that a hair on a mature seed may be up to c. 30 mm long. Basally, the seeds have a tail- to ± wing-like appendage, made up of only two cell layers, the exotesta of the ab- and adaxial side of the seed. This basal appendage shows the same anatomical structure as the wings of various anemochorous rubiaceous seeds. Although seed hairs of this kind are unique in theRubiaceae and — from the point of development and structure — not homologous to exotesta wings, the presence of a basal wing-like appendage suggests thatHillia, previously often placed into a tribe of its own (Hillieae), can be accommodated in theCinchoneae, a tribe in which winged, anemochorous seeds predominate. The tufts of hairs of the comose seeds ofHillia look superficially similar to those of certainAsclepiadaceae andApocynaceae (like theRubiaceae belonging to the orderGentianales). Comparisons based on literature data, however, reveal that there are striking differences in the position, development and structure of the hairs (produced at the micropylar end, initiated after fertilization, hairs unicellular, etc.).

Pollen Morphology and Ultrastructure

The study of pollen should encompass all structural and ornamental aspects of the grain. Pollen morphology is studied using LM and SEM and is important to visualize the general features of a pollen grain, including, e.g., symmetry, shape, size, aperture number and location, as well as ornamentation. TEM investigations are used to highlight the stratification and the uniqueness of pollen wall layers as well as cytoplasmic features. The following sections explain the most important structural and sculptural pollen features a palynologist should observe.

Misinterpretations in Palynology

The description of pollen ornamentation depends on three major parameters (1) the interpretations of the palynologist (which are subjective), (2) the pollen terminology applied, and (3) the magnification, resolution, and methods used.

Palynology: History and Systematic Aspects

Palynology is the science of palynomorphs, a general term for all entities found in palynological preparations (e.g., pollen, spores, cysts, diatoms). A dominating object of the palynomorph spectrum is the pollen grain. The term palynology was coined by Hyde and Williams (1955; Fig. 1). It is a combination of the Greek verb paluno (пαλύνω, “I strew or sprinkle”), palunein (пαλύνeιν, “to strew or sprinkle”), the Greek noun pale (пαλƞ, in the sense of “dust, fine meal,” and very close to the Latin word pollen, meaning “fine flour, dust”), and the Greek noun logos (λογος, “word, speech”).

Methods in Palynology

Multiple methods and techniques should be used when investigating pollen grains in order to provide comprehensive and accurate information about pollen morphology and ultrastructure (see also “Misinterpretations in Palynology”). The preparation methods used depend on the material to be studied, if the pollen grains are to be obtained from recent flower material (herbarium sheets, newly collected) or from various sedimentary rocks, sediments or soils (fossil to subfossil pollen). Recent and fossil pollen grains are easily studied using both LM and SEM, but recent pollen grains are also more often studied using TEM.

How to Describe and Illustrate Pollen Grains

For the description of a pollen grain, a number of features are used including size, polarity and shape, aperture condition, ornamentation, and pollen wall structure. Additional and often more specialized features depend on the group of plants under study, Gymnosperms (Cycadales, Ginkgoales, Pinales, Gnetales) vs. Angiosperms (magnoliids, monocots, commelinids, eudicots). These features can only be obtained by the application of a combined analysis with LM, SEM, and TEM (Fig. 1). In order to compare and categorize pollen, a common language and understanding of technical terms is necessary.

Glossary of Palynological Terms

All important terms in palynology are listed here and explained. Terms figured in chapter “Illustrated Pollen Terms” are indicated by bold page numbers. Non-recommended terms are only provided with an explanatory comment. For consistency, phrases are standardized, for example, features of ornamentation are stereotypically defined as “pollen wall with …”, and pollen wall features (or pollen shape and size) as “pollen grain with …”.